The present invention relates to an objective optical system for an optical information recording/reproducing apparatus adapted to record information to and/or reproduce information from a plurality of types of optical discs based on different standards, and to an optical information recording/reproducing apparatus on which the objective optical system is mounted.
There exist various standards of optical discs, such as DVD (Digital Versatile Disc) and BD (Blu-ray Disc), differing in recording density, protective layer thickness, etc. Therefore, the objective optical system mounted on the optical information recording/reproducing apparatus is required to have a compatibility with the plurality of types of optical discs. The “compatibility” as used herein means to guarantee realizing information recording and information reproducing without the need for replacement of components even when the optical disc being used is changed.
In order to achieve the compatibility with the plurality of types of optical discs based on the different standards, it is necessary to correct the relative spherical aberration caused by the difference in protective layer thickness between the plurality of types of optical discs and to form a suitable beam spot for each of the plurality of types of optical discs in accordance with the difference in recording density between the plurality of types of optical discs, by changing the numerical aperture NA of the objective optical system used for information recording or information reproducing. The optical information recording/reproducing apparatus is configured to selectively use one of laser beams having different wavelengths respectively corresponding to recording densities of the plurality of types of optical discs. That is, the optical information recording/reproducing apparatus selectively uses one of the laser beams in accordance with the recording density of an optical disc being used. The optical information recording/reproducing apparatus uses, for example, light having the wavelength of approximately 790 nm (i.e., near infrared laser light) for information recording or reproducing for CD, light having the wavelength of approximately 660 nm (i.e., red laser light) for information recording or reproducing for DVD and light having the wavelength of approximately 405 nm (i.e., blue laser light) for information recording or reproducing for BD.
Each of Japanese Patent Provisional Publication No. 2006-164498A (hereafter, referred to as JP2006-164498A) and International Publication No. WO 2008/007552 discloses an optical information recording/reproducing apparatus having the compatibility with three types of optical discs.
The optical information recording/reproducing apparatus disclosed in JP2006-164498A has an objective optical system provided with a diffraction structure configured such that the diffraction order at which the diffraction efficiency is maximized for the blue laser light is an even order so that the diffraction efficiency can be enhanced for the near infrared laser light. In order to correct the spherical aberration when the near infrared laser light is used, it is necessary to cause the near infrared laser light to be incident on the objective optical system as a diverging beam. In this case, it is impossible to avoid occurrence of off-axis aberrations, such as a coma, when the objective optical system is shifted by a small amount in a direction orthogonal to an optical axis of the objective optical system for a tracking operation.
The optical information recording/reproducing apparatus disclosed in WO 2008/007552 is configured to cause each of the blue laser light and the red laser light to be incident on the objective optical system as a collimated beam, and to cause the near infrared laser light to be incident on the objective optical system as a collimated beam or a diverging beam. In WO 2008/007552, the objective optical system is designed to try to suppress deterioration of a signal due to flare light by appropriately setting converging points of two types of orders of diffracted light caused in a central region of an objective optical element. However, since the converging positions of the two types orders of diffracted light are close to each other, the configuration disclosed in WO 2008/007552 is not adequate for the measure against the flare light. Furthermore, since a peripheral region of the objective optical element provided as a dedicated region for the blue laser light is a refractive surface, it is impossible to control the spherical aberration caused, for example, by the wavelength variation of laser light or the temperature change.